1. Field of the Invention
The present invention relates to an optical pickup device for converging a light beam from a semiconductor laser onto an optical disc serving as a storage medium. More specifically, this invention relates to an optical pickup device which detects a quantity of a light beam emitted from a semiconductor laser, by means of a front monitor, to perform a power control of the light beam.
2. Description of the Prior Art
In general, an optical pickup for recording information signal onto a storage medium such as an optical disc is equipped with a semiconductor laser having a light emission power of 30 mW order. As a method of controlling the power of the light beam emitted from the semiconductor laser, there are known two methods: a front monitor method and a rear monitor method. In the front monitor method, a part of the light beam emitted from the semiconductor laser toward the storage medium is detected by a light detector, and the detected signal is fed back to a drive circuit of the semiconductor laser so as to control the power of the light beam emitted from the semiconductor laser to be a predetermined level. On the other hand, in the rear monitor method, the light beam emitted from the backface of the semiconductor laser, i.e., the light emission surface opposite to the light emission surface for emitting the light beam toward the storage medium, is detected, and the power of the light beam from the semiconductor laser is controlled in the same manner as the front monitor method. Generally, the rear monitor method is known as being unsatisfactory in the light beam detection accuracy, and hence the front monitor method has been broadly adopted.
FIG. 14 shows a schematic configuration of an optical pickup device according to a conventional front monitor method. A light beam emitted from a semiconductor laser 1 is converted into the parallel light by a collimator lens 2 and supplied to a beam splitter 4 via a grating 3. The grating 3 separates the incident light beam into three beams, i.e., a main-beam used for reading out information from the optical disc 8 and two sub-beams used for a tracking serve control. The beam splitter 4 includes a reflection film 5 which transmits approximately 90% of the light beam from the semiconductor laser 1 and reflects the remaining approximately 10% of the light beam. Namely, by the function of the reflection film 5, approximately 90% of the light beam supplied to the beam splitter 4 is transmitted therethrough to be guided to a 1/4 wavelength plate 6, and the remaining approximately 10% of the light beam is reflected by the reflection film 5 to be guided to a condensing lens 12. The light beam guided to the 1/4 wavelength plate 6 is converged on a recording surface of an optical disc 8 by means of an objective lens 7, thereby to form beam spots of predetermined sizes.
The light beams irradiated on the recording surface of the optical disc 8 are reflected by the surface and travels to the reflection film 5 of the beam splitter 4 via the objective lens 7 and the 1/4 wavelength plate 6. Since the reflection film 5 has a property to reflect approximately 100% of the light beam from the direction of the optical disc 8, the light beam incident upon the reflection film 5 is guided to the light receiving element 11 via a condenser lens 9 and a cylindrical lens 10 for giving astigmatism to the light beam. On the other hand, the approximately 10% of the light beam emitted from the semiconductor laser 1 and reflected by the reflection film 5 of the beam splitter 4 is converged on a front monitor detector 13 by a condenser lens 12. The front monitor detector 13 outputs an electric signal depending upon the quantity of the light beam irradiated thereon, and the electric signal is supplied to an automatic power control (APC) circuit 14 including a laser control circuit which controls the power of the semiconductor laser 1. The APC circuit 14 derives an appropriate drive signal for driving the semiconductor laser 1 in accordance with the electric signal from the front monitor detector 13, and supplies the drive signal to the semiconductor laser 1. Thus, the output power of the semiconductor laser 1 is controlled by the drive signal generated by the APC circuit 14 based on the electric signal outputted from the front monitor detector 13.
In order to reduce the load on the semiconductor laser, reduce the power consumption of the semiconductor laser or obtain high laser power output at the time of recording processing, it is preferred to enhance the efficiency in use of the light beam emitted from the semiconductor laser. However, the pickup device employing the conventional front monitor method described above is designed such that the approximately 10% of the light beam emitted from the semiconductor laser 1 and supplied to the beam splitter 4 is necessarily guided to the front monitor detector 13. In other words, approximately 10% of the light beam incident upon the reflection film is reflected without exception. Therefore, the efficiency in use of the light beam is degraded.
Further, the reflective and transmissive property of the reflection film provided in the beam splitter may have irregularity within about xc2x15% from product to product, and hence, if the reflectance is designed to be 10%, the reflectance of the actual product may greatly vary within the range from 5% to 15%. Therefore, the gain control of the APC circuit must be carried out for every product, thereby deteriorating the production efficiency. Furthermore, it is known that the reflectance and/or transmittance of the reflection film in the beam splitter may vary dependently upon the ambient humidity. Therefore, the conventional front monitor method, which relies on the property of the beam splitter in controlling the output power of the semiconductor laser 1, is unsatisfactory in its reliability.
The present invention is contrived in view of the above mentioned problems, and it is an object of the present invention to provide an optical pickup device capable of enhancing the efficiency in use of the light beam and stably performing the power control of the semiconductor laser without being affected by the irregularity and/or the humidity-dependent variation of the property of the reflection film employed in the beam splitter.
According to one aspect of the present invention, there is provided a pickup device including: a light source for emitting a light beam; an optical system for separating the light beam into a main-portion and a sub-portion, and for guiding the main-portion of the light beam to an information storage medium; a monitor detector for receiving the sub-portion of the light beam and for outputting a detection signal; and a controller for controlling an output power of the light beam emitted by the light source based on the detection signal.
In accordance with the optical pickup thus configured, a light beam is emitted by the light source. The optical system separates the light beam into a main-portion and a sub-portion, and guides the main-portion of the light beam to an information storage medium. The monitor detector receives the sub-portion of the light beam and outputs a detection signal. Then, the controller controls the output power of the light beam emitted by the light source based on the detection signal. Thus, the light beam of the sub-portion, which is not generally used as a light beam to be irradiated on a storage medium, can be efficiently used, and hence the output power of the light beam from the light source may be controlled with high accuracy.
Preferably, the main-portion is a center portion of the light beam and the sub-portion is a portion of the light beam other than the main portion.
The optical system may include an interrupting member for partially interrupting the light beam from the light source and passing only the main-portion and the sub-portion of the light beam. Thus, it is possible to prevent unnecessary light component from entering the monitor detector. As an example, the interrupting member may include a first aperture for passing the main-portion of the light beam and at least one second aperture for passing the sub-portion of the light beam.
Preferably, the pickup device may further include a casing for covering a light emitting part of the light source to receive whole portion of the light beam emitted by the light source, wherein the interrupting member is disposed on the casing at a position 5receiving the light beam from the light source. By this, the more reliable interruption of the unnecessary light beam is ensured.
In a preferred embodiment, the optical system may include a collimator lens, wherein the collimator lens includes a miniature convex lens formed at an edge part thereof and for directing the sub-portion of the light beam to the monitor detector. Similarly, the optical system may include a collimator lens, wherein the collimator lens includes two miniature convex lenses formed at edge parts thereof opposing to each other, and the two convex lenses directs the sub-portions of the light beams to the monitor detector.
In another preferred embodiment, the optical system may include a diffraction grating having a first grating pattern for directing the main-portion of the light beam to the storage medium and a second grating pattern for directing the sub-beam to the monitor detector. Further, the second grating pattern may be configured to direct all component of the light beam other than the main-portion to the monitor detector as the sub-portion of the light beam.
In still another preferred embodiment, the optical system may include a beam splitter having a light receiving surface and a light reflecting surface, wherein the light receiving surface guides the main-portion of the light beam to the storage medium and the light reflecting surface reflects the sub-portion of the light beam to the monitor detector.
The monitor detector may include two detection elements each for outputting an electric signal corresponding to a quantity of light received, and an adder for adding two electric signals to produce the detection signal.
According to another aspect of the present invention, there is provided a pickup device for irradiating a main-portion of a light beam emitted from a light source on a storage medium, including: a light detector for receiving a sub-portion of the light beam which is a portion other than the main-portion of the light beam emitted by the light source and outputting a detection signal; and an adjusting unit for adjusting a power of the light beam emitted by the light source based on the detection signal.
In accordance with the pickup device thus configured, the light beam of the sub-portion, which is not generally used as a light beam to be irradiated on a storage medium, can be efficiently used, and hence the output power of the light beam from the light source may be controlled with high accuracy.
The pickup device may further include a separating unit for separating the light beam emitted by the light source into the main-portion to be irradiated on the storage medium and the sub-portion to be guided to the light detector. Thus, the sub-portion of the light beam can be efficiently guided to the monitor detector. In addition, the separating unit may reform the shape of the main-portion of the light beam. By this, the light beam can be irradiated on the storage medium with high accuracy.
In a preferred embodiment, the main-portion of the light beam may include a center portion of the light beam, wherein the sub-portion of the light beam includes a component of the light beam positioned outside of the main-portion, and the pickup device further including a changing unit for changing the optical path of the sub-portion of the light beam to the light detector. Thus, the optical path of the main-portion of the light beam can be separated from the optical path of the sub-portion of the light beam, thereby facilitating the design of the optical system.
In a specific embodiment, the light detector may include at least two detection elements, each of the detection elements receiving the sub-portion of the light beam at the position sandwiching the main-portion of the light beam in a symmetrical manner. This enables downsizing of the optical pickup.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiment of the invention when read in conjunction with the accompanying drawings briefly described below.